1. Field of the Invention
The present invention relates to a process for producing polyethylene terephthalate (hereinafter referred to as "PET") useful for molding bottles, films and sheets. More particularly, it relates to a process for producing PET whereby contamination of a mold scarcely occurs during its molding and the aldehyde content of the molded product will be little.
2. Discussion of Background
PET is excellent in the mechanical strength, chemical stability, transparency, sanitation and gas barrier property. Therefore, its use as containers for beverages such as soft drinks or fruit juices has been remarkable in recent years.
For example, in a case of a stretched bottle, such PET is molded into a preform for a hollow molded product by an injection molding machine, and this preform is then subjected to stretch blow molding in a mold having a predetermined shape. In a case of a bottle for a beverage such as a fruit juice beverage which requires hot filling, it is common to improve the heat resistance by applying heat treatment so-called heat setting.
However, conventional PET chips used for molding usually contain oligomers including a cyclic trimer as the main component. Such oligomers tend to deposit on e.g. the apparatus such as the mold, thus leading to contamination of the mold. Such contamination of the mold is likely to cause blushing or surface roughening of the molded bottles. Therefore, it used to be necessary to frequently clean the mold.
Further, a molded product obtained by molding conventional PET contained a substantial amount of acetaldehyde formed during the molding. Therefore, there was a problem that such acetaldehyde tended to migrate to the content of the molded product and adversely affected the taste or the odor.
In an attempt to solve Such problems, there have been proposed a method for reducing the oligomers and aldehyde by subjecting a polyester prepolymer obtained by melt polymerization to solid-state polymerization under reduced pressure or under a stream of an inert gas (Japanese Unexamined Patent Publications No. 89330/1980 and No. 89331/1980), a method of subjecting a polyester prepolymer to moisture conditioning so that the water content will be at least 2,000 ppm, followed by crystallization and solid-state polymerization (Japanese Unexamined Patent Publication No. 219328/1984, a method of treating polyester particles with hot water of from 50.degree. to 200.degree. C., followed by heat treatment under reduced pressure or under a stream of an inert gas (Japanese Unexamined Patent Publication No. 55426/1981) and a method of applying extraction and cleaning treatments with water or an organic solvent before and after the solid-state polymerization (Japanese Unexamined Patent Publication No. 13715/1980). However, the oligomers and acetaldehyde have not adequately been reduced in the molded products prepared from polyester chips obtained by these methods.
Further, a method of treating PET with water of from 1.degree. to 150.degree. C. or with steam or a steam-containing gas has been proposed (Japanese Unexamined Patent Publication No. 215521/1991). However, such a method has problems that it is difficult to avoid reduction of the intrinsic viscosity of PET during the treatment or during a drying operation, and the heating cost required for drying tends to increase, or the productivity tends to be low, since the water content of PET after treatment is high.